1. Field of the Invention
The invention relates to a steering device that is equipped with an electric motor so as to provide steering assist force for the pivoting operation of a steering handle and a control method for the steering device.
2. Description of the Related Art
An electric power steering device according to the related art is equipped with an electric motor so as to provide steering assist force for the pivoting operation of a steering handle, and adjusts the steering assist force by controlling the electrification of the electric motor. Such an electric power steering device is supplied with source power from a vehicle-mounted electric power source device. However, in the case where an abnormality occurs in the vehicle-mounted electric power source device, the electric power steering device cannot appropriately perform the control of the electric motor. Therefore, for example, an electric power steering device proposed in Japanese Patent Application Publication No. 2005-343323 (JP-A-2005-343323) adopts a construction in which when the power source voltage of the vehicle-mounted electric power supply device declines and reaches a first reference value, the maximum drive current of the electric motor is restricted, and when the power source voltage reaches a second reference value, the motor control output gain is reduced, and when the power source voltage reaches a third reference value, the output restraint control is performed.
Besides, an electric power steering device in which the electric power supply device that supplies source power to an electric motor is constructed of two electric power sources is also known, for example, in Japanese Patent Application Publication No. 2007-91122 (JP-A-2007-91122). In the construction equipped with two electric power sources, if one of the electric power sources fails, the electric motor can still be driven by using the other electric power source so as to obtain the steering assist force.
However, in the case where one of the two electric power sources fails, if the electric motor is driven and controlled by using the other electric power source without any particular change or the like, it becomes difficult to make the driver aware of the system abnormality of the electric power steering device. For example, even if the system abnormality is indicated by turning on a warning lamp, or the like, the driver sometimes does not notice it. In such a case, if the electric motor is driven and controlled by using the other electric power source (the electric power source that is normal) without any particular change, the source power supply capability of that electric power source rapidly declines, and the system stops in an early period. Besides, the steering assist is suddenly lost, thus causing considerable discomfort to the driver.